Heat packs utilizing supercooled aqueous salt solutions have been used for some time for the treatment of soreness of muscles of athletes and sportsman in localized areas. Heat packs are used also as infant heel warmers in medical facilities as an aid in drawing blood. From the simple hot water bottle we have progressed upward to the use of supercooled aqueous salt solutions wherein not only the temperatures can be controlled but so also the duration of the heat given off. Various solutions such as sodium acetate and calcium nitrate tetrahydrate are examples of such solutions.
Various techniques of initiating crystallization have been proposed, including inserting a crystal of material into the supercooled solution, and scraping some metal inside the container to introduce impurities into the supercooled solution. Examples of these techniques are disclosed in the following U.S. Pat. Nos. 1,433,010, 2,289,425, 2,220,777, 3,093,308, 4,077,390 and 4,572,158. U.S. Pat. No. 1,915,523 discloses the introduction of air into the solution by means of a valve. This complicated method involves the use of both hands to manipulate the valve.
The present invention provides a simple device that is easily manufactured and the trigger is easily activated. Of particular advantage is that the heat pack of the present invention can be initiated using one hand. Of further advantage is that a temperature indicator can be attached to the heat pack to indicate the appropriate temperature. A final advantage is that the heat pack can be secured to a body part, thereby simplifying its use.